Conventionally, a semiconductor device has been proposed in which a semiconductor element is mounted on one surface (for example, at a front surface) of a wiring substrate, electrodes (lands) are arranged in vertical and horizontal directions in a grid state at a rear surface side, and solder balls connecting to a mounting substrate such as a printed wiring board are disposed on these electrodes (lands), so as to make high function and miniaturization of various electronic equipments easy. The semiconductor device as stated above is called as a BGA (Ball Grid Array) type semiconductor device (for example, refer to JP-A 2007-317754 (KOKAI)). The wiring substrate has a multilayer structure in which insulating layers composed of an organic material such as glass epoxy and conductive layers are layered alternately to be integrated, and an inner conductive layer is made of a copper foil to be a power plane layer or a ground plane layer.
In the semiconductor device having the structure as stated above, a major part of heat generated by an operation of the semiconductor element is conducted to the mounting substrate from the wiring substrate via the solder balls, and it is discharged from the mounting substrate. Temperatures of the solder balls positioning along a heat conductive path become high resulting from this heat conduction. The inner conductive layer constituting the wiring substrate is formed in a plane state using the copper foil of which heat conductivity is good. Accordingly, the temperatures of the solder balls disposed in a vicinity of four corners also become high, because the heat is conducted not only to an area just below the semiconductor element but also to a peripheral area at the rear surface of the wiring substrate. Kirkendall void is easy to be generated at electrode junctions of the solder balls when the temperature becomes high.
On the other hand, when the heat is added under a state in which the semiconductor device is mounted on the mounting substrate via the solder balls, a strain (thermal strain) is generated on the solder ball caused by a difference of coefficients of thermal expansion between the wiring substrate of the semiconductor device and the mounting substrate. The farther the solder ball is disposed from a center portion of the semiconductor device, the larger the thermal strain becomes. The thermal strain of the solder ball disposed in the vicinity of the corner portion therefore becomes large, and it becomes easy to break. Accordingly, an operating life becomes short resulting from fatigue.